You have the choice to put a signature as soon as you register. In the registration form, there's a section where you can add a signature.

You can't add a signature only after your account activation.

Once activated and logged in, you click on the button in the navigation links at the top of the page.

Then you click on the "Signature" tab above the frame, at the right hand side of your page.

Then you fill the signature field and you can use the BBCode buttons above the field to add effects, links, images or flash animations.

After you filled up your sign field and saved, return to your profile edition form, click on the "Preferences" link just above the frame and then, make sure that you ticked "Yes" to the "Always attach my signature" option.